tests/test-execute-bit.t

pathutil: add doctests for canonpath()

This is a followup to f445b10dc7fb.  Since there's no way to ensure that more
drive letters than C: exist, this seems like the only way to test it.  This is
enough to catch the f445b10dc7fb scenario, as well as CWD outside of the repo
when the path isn't prefixed with path/to/repo.

#require execbit

  $ hg init
  $ echo a > a
  $ hg ci -Am'not executable'
  adding a

  $ chmod +x a
  $ hg ci -m'executable'
  $ hg id
  79abf14474dc tip

Make sure we notice the change of mode if the cached size == -1:

  $ hg rm a
  $ hg revert -r 0 a
  $ hg debugstate
  n   0         -1 unset               a
  $ hg status
  M a

  $ hg up 0
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg id
  d69afc33ff8a
  $ test -x a && echo executable -- bad || echo not executable -- good
  not executable -- good